CARET is an ongoing double-blind lung cancer chemoprevention trial of the efficacy and safety of the combination of 30 mg beta-carotene and 25,000 IU retinyl palmitate daily in two high-risk populations: (a) male and female current and former smokers recruited primarily from insurance-based sources, and (b) males with extensive occupational asbestos exposure recruited primarily from employment-based sources. We have documented successful recruitment, excellent compliance, and minimal side effects in over 15,000 participants randomized thus far at six study centers. Through 30 April, 1993, CARET has randomized 4,000 asbestos-exposed workers, exceeding accrual goals at all five CARET asbestos centers, and 11,105 heavy smokers. Seattle and Portland have exceeded their heavy smoker goals, and Irvine is on track to achieve its goal in July 1994. During the final five-year period, CARET will focus on retention, adherence to protocol, ascertainment of endpoints, monitoring of key design parameters, closeout of the trial, and analysis and publication. Projections indicate that with 14,240 smokers and 4,010 asbestos-exposed participants and 114,100 person-years of follow-up through February 1998, CARET will be capable of detecting a 23% reduction in lung cancer incidence in the two high-risk populations combined, and 27%, 49%, 32%, and 35% reductions in the smokers, female smokers, male smokers, and asbestos-exposed subgroups, respectively. The Portland Study Center will conduct follow-up activities with 4,200 participants randomized into the heavy smoker cohort, 280 participants randomized into the asbestos-exposed worker cohort, and 32 Vanguard transfer participants. High adherence and retention and complete ascertainment of endpoints and other data will be primary goals. Clinical, data management, and follow-up activities include annual and semiannual clinic visits; routine telephone interviews; symptom, adherence, ancillary variables, and study endpoint documentation and reporting; quality assurance of data collection; and other tasks according to the established CARET protocol and procedures. The investigators will collaborate with the Coordinating Center and other Study Centers on the analyses and publication of data.